Re: [Qemu-devel] [PATCH v2 10/11] tcg/mips: Make direct jump patching thread-safe

[Aurelien Jarno <aurelien@aurel32.net>]

On 2016-04-22 20:00, Sergey Fedorov wrote:
> On 22/04/16 19:51, Aurelien Jarno wrote:
> > On 2016-04-22 18:47, Aurelien Jarno wrote:
> >> On 2016-04-22 19:08, Sergey Fedorov wrote:
> >>> From: Sergey Fedorov <serge.fdrv@gmail.com>
> >>>
> >>> Ensure direct jump patching in MIPS is atomic by using
> >>> atomic_read()/atomic_set() for code patching.
> >>>
> >>> Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
> >>> Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
> >>> ---
> >>>
> >>> Changes in v2:
> >>>  * s/atomic_write/atomic_set/
> >>>
> >>>  tcg/mips/tcg-target.inc.c | 3 ++-
> >>>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>>
> >>> diff --git a/tcg/mips/tcg-target.inc.c b/tcg/mips/tcg-target.inc.c
> >>> index 682e19897db0..cefc0398018a 100644
> >>> --- a/tcg/mips/tcg-target.inc.c
> >>> +++ b/tcg/mips/tcg-target.inc.c
> >>> @@ -1886,6 +1886,7 @@ static void tcg_target_init(TCGContext *s)
> >>>  void tb_set_jmp_target1(uintptr_t jmp_addr, uintptr_t addr)
> >>>  {
> >>>      uint32_t *ptr = (uint32_t *)jmp_addr;
> >>> -    *ptr = deposit32(*ptr, 0, 26, addr >> 2);
> >>> +    uint32_t insn = atomic_read(ptr);
> >>> +    atomic_set(ptr, deposit32(insn, 0, 26, addr >> 2));
> >>>      flush_icache_range(jmp_addr, jmp_addr + 4);
> >> Does it really make sense to read and write the value atomically? The
> >> resulting operation is still not atomic, something can happen in
> >> between.
> > Hmm, thinking more about that, given the only instruction used is "J",
> > we don't have to read the value, patch it and write it. We can directly
> > use something like (untested):
> >
> >     atomic_set(ptr, (0x02 << 26) | (addr >> 2));
> 
> Hmm, looking at "case INDEX_op_goto_tb:" in tcg_out_op() again I'm
> thinking about:
> 
>     atomic_set(ptr, deposit32(OPC_J, 0, 26, addr >> 2));

Oh right, I forgot when reviewing the patch that this code is actually
in tcg-target.inc.c, and that the OPC_J constant is available.

Your version looks good to me.

Aurelien

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien@aurel32.net                 http://www.aurel32.net